Metal implants are widely used both in orthopedic hip and knee surgeries and in dental surgery. Over 2 million orthopedic procedures and over 10 million dental implant procedures are performed in the United States every year. Implants fail because of poor osseointegration between the implant and the natural bone. Therefore, for the implants to function successfully, a direct chemical bond between the implant and the bone needs to form rapidly and needs to be retained over many years while the implant is loaded. Metal, however, does not form a direct chemical bond with bone. In order to promote osseointegration between the metal implant and bone, a layer of calcium phosphate ceramic material is coated onto the implants.
In current practice, calcium phosphate coatings are repeatedly applied using the mechanical and physical forces of plasma spraying. This method involves firing high temperature and high energy molten droplets of calcium phosphate at a high velocity onto the surface of the implant where they stick and freeze. Plasma sprayed calcium phosphate coatings have many disadvantages, though. Chief among these drawbacks is the inability to form thin coatings on the order of 25 microns or less. The process generally produces layers that are about 50 microns thick, remain on the implant for very long periods of time, and tend to delaminate under load, resulting in implant loosening. The slow resorption of calcium and the inability of bone cells to penetrate the thick, dense calcium phosphate layer cause the implant and calcium phosphate interface to weaken.
Accordingly, a need exists for a method of making a thinner biocompatible layer of calcium phosphate on metal implants for use in orthopedic and dental applications.
The present invention thus provides a low energy method of coating thin layers of biocompatible calcium phosphate onto metal implant surfaces. The novel method uses a biomimetic mechanism to produce thin, resorbable layers of calcium phosphate. These biomimetic calcium phosphate coatings may also be reinforced with hydrogel polymers to improve their mechanical strength. The invention also describes methods to immobilize bone inducing growth factors onto reactive hydrogel polymers that may accelerate the fixation of the implant to the bone. Other inventive features and advantages will become apparent from the following detailed description, examples, drawings and claims.